Motors, Motor Circuits and Motor Controllers

Thanks for your great responses to our new Code FAQs column! This month, as promised, this column addresses inquiries relating to motors, motor circuits and motor controllers. Again, if you have a National Electrical Code (NEC)-related problem, are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, send your questions to me at letthecodedecide@earthlink.net.

This has always bothered me. Why do they require the motor-circuit conductors to be sized at 125 percent of the full load current of a motor and then permit these conductors to be protected by overcurrent devices that are, in most cases, much higher than the ampere (A) rating of the conductor?

Article 430.22 requires the motor branch-circuit conductors to have an ampacity of not less than 125 percent of the motor’s full-load current rating for two reasons. Separate motor-overload protective devices are, in most cases, permitted to be rated at 125 percent of the motor’s full-load current rating. This means a motor can run continuously with an overload of about 125 percent. Sizing the motor branch-circuit conductors at 125 percent of the motor full-load current will provide protection for the motor circuit conductors if this should occur. Article 430.52 permits the motor branch-circuit short circuit and ground-fault protective devices to be sized according to Table 430.52. The Code requires these devices to carry the starting current of the motor.

Remember that 430.6(2) requires the motor-overload devices to be sized according to the motor nameplate full-load current rating, and 430.6(1) requires the motor branch-circuit short circuit and ground-fault devices to be sized according to Tables 430.247, .248, .249 and .250, respectively.

Here are some tips for motor-overload protection: In accordance with 430.36, where fuses are used for motor-overload protection, a fuse must be inserted in each ungrounded conductor and also in the grounded conductor, if the supply system is a three-phase, 3-wire air conditioner with one conductor grounded. Where devices other than fuses are used, 430.37 requires the use of Table 430.37 to determine the number and location of overload units, such as trip coils or relays.

Are motor-control circuits feeding remote control devices required to have overcurrent protection?

Article 430.72(B) covers overcurrent protection for motor-control circuit wiring. The requirements for conductors that extend beyond the enclosure (remote) are located in Column C of Table 430.72(B). For example, if your motor branch-circuit protective device is rated at 60A and you are using copper control-circuit conductors, you find 60 in the copper column and move to the left to control-circuit conductor size, where you find 12. This means that you need to install control-circuit conductors not smaller than 12 AWG copper. A smaller conductor would require supplemental overcurrent protection to protect that conductor.

Can you explain how, after using Column C in Table 430.72(B), you can tell that a 60A overcurrent device will protect a 12 AWG copper control-circuit conductor?

The motor control-circuit conductors are protected by the raceway in which they are installed. The danger of physical damage is minimal. The coil circuit current is small, and overload is not a problem. Short-circuit or ground-fault currents rise rapidly and would open the motor branch-circuit protective devices before any damage could occur.

If I tap a motor feeder supplying a group of motors, how far can the tap be run without overcurrent protection for the tap conductors?

If you are running a tap to a single motor and are not reducing the conductor size, overcurrent protection is not a problem. However, if you are reducing the conductor size, 430.53(D)(2) requires that no conductor to the motor can have an ampacity less than one-third of that of the feeder with a minimum in accordance with 430.22, the conductors to the overcurrent device being not more than 25 feet long and being protected from physical damage. This means the tap conductor must be sized at 125 percent of the motor full-load current shown on the tables and be not less than one-third the ampacity of the feeder conductor.

The electric motor I am installing is marked SF 1.15. I know it’s the service factor, but what does this mean?

Yes, SF is the service factor. Service factor is a margin of safety. If the manufacturer gives a motor a service factor, it means that the motor can develop more than its rated horsepower without damage to the motor. A 5 horsepower motor with a service factor of 1.15 can develop 5.75 horsepower current without damage to the motor insulation. Motors with a continuous-duty rating as shown in 430.32 can operate indefinitely at their rated load. A continuous-duty motor with a marked service factor of 1.15 or greater can, as shown in 430.32(A)(1), be overloaded 25 percent without damage to the motor.

How is the motor-disconnecting means sized?

NEC 430.110(A) requires that the motor-circuit disconnecting means (600 volts or less) have an ampere rating not less than 115 percent of the motor full-load current taken from Table 430.247, 430.248, 430.249 or 430.250.

Can the motor-controller disconnecting means be used as the motor-circuit disconnecting means?

Yes, 430.102(B) permits the motor-controller disconnecting means to serve as the motor-circuit disconnecting means if it is in sight from the motor location and the driven machinery location. The exception to 430.102(B) allows the disconnecting means to be out of sight from the motor if the disconnecting means is capable of being locked in the open position.

How do I determine the rating of the overcurrent device used in the motor-disconnecting means?

No overcurrent-protective devices are required in the motor-circuit disconnecting means. This disconnecting means is solely for the protection of people maintaining the motor or the driven machinery, and this disconnect is not required if the motor-controller disconnecting means is capable of being locked in the open position (430.102(B)).

Are motor branch-circuit conductors sized at 125 percent of the motor full-load current able to handle the motor starting current?

No. The motor starting or inrush currents, also called “locked rotor current,” are only present during the acceleration period at the moment a motor is started. The inrush current decreases rapidly as the motor begins to rotate. The motor branch-circuit protection calculated from Table 430.152 easily handles these currents within the limitations of the motor branch-circuit conductors. The motor branch-circuit overcurrent protective devices, which are permitted to be sized much higher than the rated ampacity of the motor branch-circuit conductors, are also able to protect the motor branch-circuit conductors from short-circuit or ground-fault currents because of the magnitude of the currents produced so rapidly by these types of faults.

Can a sump pump located in an elevator pit use a cord and cap plugged into a receptacle as its disconnecting means?

A sump pump located in the elevator pit is permitted by 620.21(A)(1)(d)(e) to be cord-connected. A single receptacle supplying a permanently installed sump pump does not require ground-fault circuit interrupter protection (620.85). The attachment plug and receptacle is permitted by 430.109(F) to serve as the disconnecting means.